In prior art linear actuators, the electric motor, supported on a housing, rotationally drives a ball screw shaft formed with a ball screw. The ball screw shaft axially drives an output member connected to a ball nut engaging the ball screw shaft. Friction on the ball screw is very low. Thus, the ball screw shaft can easily be rotated by an axial load acting on the output member. The linear actuator is incorporated with mechanism for holding a position of the output member, such as a brake of the electric motor or a power transmitting apparatus, using a worm gear with low mechanical efficiency.
It is necessary to use an electric motor with a large output in order to provide the motor with a braking function. Additionally, a continuous electric current must be supplied to the motor to generate a braking torque.
Furthermore, it is necessary to use a worm gear with a small lead angle when using a worm gear as a driving gear devoid of reverse rotation. However, the efficiency of a worm gear having a small lead angle is poor. Thus, the use of the ball screw becomes meaningless.
In Reference Patent Document 1 mentioned below, an electrically driven actuator is disclosed where an annular member is rotationally supported within a housing via bearings. A one-way clutch is arranged between the inner circumference of the housing and the outer circumference of the annular member. A ball screw shaft frictionally engages the annular member via friction members.
When the ball screw shaft is positively rotated by an electric motor, the one-way clutch is unlocked and the ball screw shaft is smoothly rotated to axially displace an output member, via a ball nut against an axial load. When the electric motor is stopped, a torque for rotating the ball screw shaft in a reverse direction acts on the ball screw shaft by the axial load acting on the output member. Although this torque acts on the annular member via the friction member, the annular member cannot rotate due to locking of the one-way clutch. Furthermore, the ball screw shaft frictionally engaging with the annular member, via the friction member, is prevented from rotating in a reverse direction by a frictional force acting on frictionally engaging portions between side faces of the friction member and their mating surfaces. When the electric motor is rotated in a reverse direction, the one-way clutch is maintained in a locked condition. However a torque transmitted from the electric motor is applied to the frictionally engaging portion in addition to the torque caused by the axial load. As a result, a slippage is caused in the frictionally engaging portions and the ball screw shaft is rotated. Thus, it is possible to displace the output member in the axial load direction.
According to the prior art electrically driven linear actuator, it is possible to hold the output member stationary against the axial load during stoppage of the electric motor as described above. In addition, continuous supply of electric current to the electric motor is not required in order to generate the braking torque in the electric motor. Additionally, the use of an electric motor having a specially large output is not required. Reference Patent document 1: Japanese Laid-open Patent Publication No. 291480/2005.
However, in the prior art electrically driven linear actuator, the positioning of the output member during stoppage of an electric motor is basically based on frictionally engaging force of the friction member and thus is unstable. Although it is possible to arrange the position holding mechanism on the ball screw mechanism itself, the space for arranging the position holding mechanism is limited due to circulation of the balls.